1. Field of the Invention
The present invention relates to surveillance systems that issue alarm signals, and, more particularly, to reducing the issuance of false alarm signals by such surveillance systems.
2. Description of the Related Art
Surveillance systems, also known as security systems, include security devices such as motion detectors for monitoring interior portions of a secured area of space, and door sensors and window sensors for monitoring perimeter portions of the secured area of space. When one of these sensors detects motion and/or the opening of a monitored door or window, the security system may issue an alarm signal that causes a siren to produce an audible alarm, and that is electronically communicated to a security company. The security company typically notifies the police, who may then visit the secured area of space in order to investigate.
A problem is that many of the alarm signals issued by a security system are what are known as “false alarms”. False alarms are not the result of a genuinely dangerous condition, such as the presence of an intruder, but rather are a result of a resident of the building moving within the secured area of space and inadvertently causing an alarm signal to be issued. For example, a resident may, without knowing or remembering that the alarm system has been activated, open a door in order to let a dog outside, or walk into a ground floor area that is monitored while the residents sleep on an upper floor. The resulting audible alarm produced by a siren as a result of a false alarm typically wakes many sleeping people needlessly. Moreover, investigations of the false alarms by the police are a waste of community resources and may result in the owners of the security system being monetarily fined.
Another problem is that even if the alarm signal is caused by a would-be intruder, it may not be desirable to notify the police immediately. For instance, the would-be intruder may first need to gain access to the premises in order to rightly be considered a definite threat that is worthy of notifying the police about. The would-be intruder may open a window or door from the outside in order to gain access to the premises, just as a resident may open a door from the inside in order to let a dog out, as mentioned above. However, the would-be intruder opening the door or window alone may not qualify as a definite threat if the residents of the building are not in the building at the time. That is, after opening the door or window, the would-be intruder may discover that the premises is protected by the security system and then leave. The homeowner may indeed want the police to investigate, but it may not be necessary for the police to be called on to respond immediately, as may be necessary in a definite threat situation. Rather, it may be desirable for the police to be notified, and for the investigation to be conducted, after the residents have returned home, and at a time that is more convenient for both the police and the homeowner.
If, however, after opening the door or window, the would-be intruder climbs through the window and proceeds through a room or hallway that is monitored by a motion detector, it may be considered a definite threat worthy of notifying the police about. It may then be desirable for the security system to issue an alarm signal.
An approach to reducing the false alarm problem described above is known as “cross zoning.” In cross zoning, two zones are said to be “crossed” when their individual areas of protection overlap one another. The system is installed and configured such that these two overlapping points are paired, i.e., “crossed”. If only one of the detection devices is faulted and the other one is not faulted, the system considers this to be a false alarm condition and will not issue an alarm signal. When both of the “crossed zones” provide a fault condition, the system issues an alarm signal.
Another approach is known as “sequential verification,” which is similar to cross zoning except that the two fault conditions that result in an alarm signal can originate from any two sensors in the security system. A single fault condition alone cannot cause an alarm signal to be issued. However, any subsequent fault condition after the first fault condition causes the alarm signal to be issued. This alarm response is sometimes referred to as a “confirmed” alarm, or, more precisely, a “sequentially confirmed alarm.”
A problem with both the cross zoning approach and the sequential verification approach is that they do not differentiate between fault conditions in different types of sensors, and they do not differentiate between a Stay mode and an Away mode of security system operation when deciding whether to issue an alarm signal. More particularly, it may not be desirable to issue an alarm signal in response to only two fault conditions from an interior sensor. Rather, it may be more desirable to issue an alarm signal only after three or more fault conditions from an interior sensor. As another example, it may not be desirable to issue an alarm signal based upon fault conditions from a single door sensor alone. This is because the door may be repetitively opening and closing due to breezes, which would cause the door sensor to produce many fault conditions. Such multiple fault conditions from the door sensor should not by themselves cause an alarm signal to issue. Moreover, it may not be desirable to wait for a second fault condition from another type of sensor before issuing an alarm signal. For example, when there are people in the building whose safety may be at risk, it may be desirable to issue an alarm signal immediately after a single initial fault condition from a sensor that monitors a window that is not normally opened.
Another problem associated with the cross zoning approach is that setting up and configuring a traditional cross zone control requires significant time and effort, which makes the cross zone control less likely to be used. For example, an installer may be required to explicitly indicate which sensors are paired together to form a “cross zone”. If the cross zone control is not used, then the goal of reducing false alarms is not realized.
What is needed in the art is a security system, and method of operation therefor, that provides a more sophisticated approach to deciding whether an alarm signal should be issued in response to various types and numbers of fault conditions. Yet, the security system and method of operation need to be simple to implement.